(1) Field of the Invention
The present invention relates to quantum cryptography (QC) apparatus and to a method of operating such apparatus. In particular, the invention relates to an apparatus and a method for synchronising the transmitter and receiver portions of a QC system.
(2) Description of the Art
The purpose of cryptography is to transmit information in such a manner that access to it is restricted entirely to the intended recipient(s). Cryptography thus allows sensitive information to be securely transmitted over public channels, such as the internet or mobile telephone networks, even if the transmission itself is received by others.
In the early days of cryptography, the security of encrypted information depended on the secrecy of the encryption and decryption procedures. However, it is now common practise to use ciphers in which the information to be encrypted (often termed the “plaintext”) and a “key” are supplied to an encryption algorithm. In such a secret key arrangement, the encryption and decryption algorithms are publicly announced and the security of the system depends entirely on the secrecy of the key. The main practical problem associated with such a secret key system is the secure distribution of the keys.
Quantum cryptography is also known and a QC system generally comprises a transmitter and a receiver, typically designated “Alice” and “Bob” respectively, that are linked by a quantum channel and a traditional (e.g. public) channel. The quantum channel is used to establish identical random number keys at the transmitter and receiver and takes advantage of the fact that, according to Heisenberg's uncertainty principle, the measurement of a quantum state will generally disturb that state. Once the secret key has been distributed using the quantum channel, encrypted information can be securely passed between Alice and Bob over the public channel.
In typical quantum communication techniques, Alice is arranged to produce a stream of single photons that are modulated randomly to one of a number of polarisation states (e.g. 0, 45, 90 or 135). Each photon can thus carry a bit of quantum information, which is often termed a qubit. Bob is arranged to measure the photons received from Alice using a random sequence of polarisation bases (e.g. a rectilinear basis and a diagonal basis). Bob then tells Alice over the public channel the bases used in the measurement and Alice then informs Bob, again over the public channel, the bases that were correctly chosen. Bob is then able to discard all the measurements where the polarisation base he used to measure the received photon does not match the polarisation base used by Alice to modulate the photon. In this manner, Bob and Alice only disclose the polarisation bases used for each measurement and are left with perfectly matching strings of bits without disclosing the results of the measurements.
An eavesdropper, typically termed Eve, attempting to gain information about the key by intercepting photons transmitted from Alice to Bob has no way of knowing the polarisation state applied to each photon by Alice. Eve must therefore randomly choose bases for her measurements and has no way of knowing, and thus cannot accurately recreate, the polarisation state of the photons transmitted by Alice. An eavesdropper will thus unavoidably corrupt the information passed to Bob by the act of measuring the polarisation of the photons transmitted by Alice. Alice and Bob can thus, as a check, choose some bits at random to reveal to one another. If the bits agree, they can use the remaining bits with the assurance they have not been intercepted. However, if they find a substantial number of discrepancies it indicates the presence of an eavesdropper and the key is discarded. The use of Quantum communication thus allows a secret key to be communicated with total security.
Following the above it can be seen that, in order for a QC system to operate effectively, it is preferably for each detection event to be matched with the corresponding transmission event. In other words, the system should be arranged so that Bob knows when a single photon pulse is expected to arrive from Alice. To reduce the impact of false detections arising from noise, the receiver of a QC system is thus typically gated so that detection events are discarded unless they occur within a period of time in which reception of a photon from the transmitter is expected.
Previously, gating of the receiver has been achieved by using a so-called “bright pulse” scheme. In such a scheme the single photon pulse is mixed with a bright pulse of a different wavelength. The receiver comprises a dichroic filter to separate the bright pulse from the single photon pulse. The bright pulse is passed to a detector (which is quite separate to the single photon detector used to detect the weak pulse) and arrival of the bright pulse is used to determine the expected arrival time of the weak pulse. However, the implementation of such a scheme adds to the complexity, and cost, of both the transmitter and receiver portions of a QC system.